The instant invention is directed to cement compositions which can be used to form novel low density cement slurries giving hardened cements with high compressive strength. This invention is particularly suitable for use in cementing operations in subterranean wells, especially oil and gas wells.
It is often necessary in the drilling and treatment of wells extending from the surface into subterranean formations to utilize a cement composition for various purposes, such as primary cementing, squeeze cementing, and plugging back. More particularly, cements are used for primary cementing involving sealing the annulus between the well casing and the wall of the hole that is bored in forming the well. In such instance a cement slurry is formed and pumped into the confined space usually through the casing from the bottom of the well. Squeeze cementing is also utilized to cure defects that may occur in primary cementing or in making a repair if there has been some damage to the well in a particular location or in other remedial work such as cementing of perforations or of channels in back of the casing. Plugging back involves sealing of the lowest portion of a well.
In pumping the cement slurry, which usually has a density in excess of 15 lbs/gal., into the bore hole for such purposes, high hydrostatic heads are caused by the weight of the cement slurry and a high pumping pressure is required which often act to cause uncontrolled fracturing of formations through which the well is bored. Such high density cements and high pump pressures can also result in loss of cement into porous types of formations.
Recognizing the problems of high density cement slurries, efforts have been made to reduce the density below 15 lbs/gal., as by adding larger amounts of water to the slurry or by including other materials in the slurry which act to lower the density. However, it has been found that the addition of water, while lowering the density, does have the undesirable effect of lowering the compressive strength of the cement, when set, below desirable levels.
With respect to density-lowering additives, a large number of materials have been used such as bentonite, attapulgite, gilsonite, diatomaceous earth, expanded perlite, and certain types of particulate carbon additives such as ground anthracite coal, lignite coal, coal cokes and bituminous coal.
However, even with such additives, while densities lower than 15 lbs/gal. can be obtained in some instances, satisfactory results are still not realized because of the deficiencies of these additives. For example, as to bentonite, if used in a proportion sufficient to lower density to the levels desired it forms a gelatinous mass that is substantially unpumpable. With gilsonite there is the high cost and softness which results in fast setting to give initial strength, but not adequate strength over time; i.e., after seven days. Others of the additives require a higher water demand in order to be pumpable and thus there is again the problem of unsatisfactory compressive strength. These additives cannot be added in sufficient quantities to lower the density to a satisfactory level and to be able to obtain rapid strength development as required in subterranean wells particularly in cementing of oil and gas wells. Also, there is the problem of high cost with many of the additives, such as the coals and the fact that coals contain high levels of volatiles that can decrease the strength of the set cement over time. Also, there is none that can give the properties desired at a density of 11 lbs/gal.
Thus, there remains the problem of obtaining an inexpensive, lightweight cement composition that can be used to form a low density cement slurry, one as low as about 11 lbs/gal. that can be readily pumped at low pump pressures to avoid fracturing of underground formations, but that will rapidly give adequate compressive strength when set even at densities as low as about 11 lbs/gal. In this latter regard, for example, the regulations of the Texas Railroad Commission require that the cement, when utilized in shallow oil and gas wells, must have a compressive strength of 250 psi at 80.degree. in 24 hours.
In addition to the foregoing requirements, because of the large volumes of materials utilized, it is also necessary to use an inexpensive additive to make the lightweight cement in order to make it economically feasible to use the same.